1. Field of the Invention
The present invention relates to a hard magnetic alloy having a supercooled liquid region, and particularly to a hard magnetic alloy which has excellent hard magnetism at room temperature and which can be formed to a bulk permanent magnet comprising a sintered or cast product. The present invention also relates to a stepping motor and a speaker using the hard magnetic alloy.
2. Description of the Related Art
Some types of multi-element alloys are conventionally known to have a wide supercooled liquid region before crystallization and constitute glassy alloys. It is also known that such glassy alloys can be formed to bulk alloys significantly thicker than amorphous alloy ribbons produced by a conventional known liquid quenching method.
Examples of conventional known amorphous alloy ribbons include ribbons of Fe--P--C system amorphous alloys first produced in the 1960's, (Fe,Co,Ni)--P--B system and (Fe,Co,Ni)--Si--B system alloys produced in the 1970's, and (Fe,Co,Ni)--M (Zr,Hf,Nb) system alloys and (Fe,Co,Ni)--M (Zr,Hf,Nb)--B system alloys produced in the 1980's. All these alloys must be produced by quenching at a cooling rate in the 10.sup.-5.degree. C./s level, and the produced ribbons have a thickness of 50 .mu.m or less.
On the other hand, glassy alloys having a thickness of several millimeters are obtained, and such glassy alloys include alloys having the Ln--Al--TM, Mg--Ln--TM and Zr--Al--TM (wherein Ln represents a rare earth element, and TM represents a transition metal) system compositions and the like, which were discovered in 1988 to 1991.
However, all these conventional known glassy alloys have no magnetism at room temperature, and from this viewpoint, these alloys are industrially greatly restricted when considered as hard magnetic materials.
Therefore, if a thick alloy comprising an amorphous single phase is obtained, the crystal structure is made fine and uniform after heat treatment, and good magnetic properties are expected. Therefore, research and development have conventionally proceeded with respect to glass alloys having hard magnetism at room temperature and permitting the formation of thick bulk products. These alloys having various compositions exhibit a supercooled liquid crystal liquid state at room temperature, but the temperature width .DELTA.Tx of the supercooled liquid region, i.e., the difference (Tx-Tg) between the crystallization temperature (Tx) and the glass transition temperature (Tg), is generally small. Therefore, in fact, such alloys have the low ability to form glassy alloys, and are thus unpractical. In consideration of this, the presence of an alloy having a supercooled liquid region having a wide temperature width, and capable of forming a glassy alloy by cooling overcomes thickness restrictions of conventional known amorphous alloy ribbons, and such an alloy thus attracts much attention in the metallurgical field. However, whether or not such an alloy can be developed as an industrial material depends upon the finding of a glassy alloy exhibiting ferromagnetism at room temperature.
Also examples of conventional known magnet materials having performance superior to ferrite magnets include Sm--Co sintered magnets, Fe--Nd--B sintered magnets, Fe--Nd--B quenched magnets, and the like. In order to achieve higher performance, there are many researches on new alloy magnets such as Fe--Sm--N magnets.
However, these magnet materials must contain 10 atomic % or more of Nd or 8 atomic % or more of Sm, and thus have the drawback that the production cost is higher than the ferrite magnets because a large amount of expensive rare earth element is used. The ferrite magnets are produced at lower cost than these rare earth magnets, but have insufficient magnetic properties. Therefore, there is demand for appearance of a magnet material costing less and exhibiting hard magnetism higher than ferrite magnets.
On the other hand, as a magnet generally known as a "bonded magnet", a magnet formed by compression molding or injection molding a mixture of a magnetic powder and a rubber or plastic binder can widely be used as electronic parts because of high shape freedom, but has the problems of low magnetic performance due to low remanent magnetization, and low material strength because of inclusion of the binder.
Possible applications of these magnets include a stepping motor and a speaker.
The stepping motor is a special motor in which the rotation can be arbitrarily controlled by a pulse current. Therefore, the stepping motor requires no feedback control, is capable of positioning in an open loop, and used as a drive source in a positioning control system in various fields. Particularly, since a hybrid stepping motor has high rotational torque and is small and capable of performing precise positioning control, the hybrid stepping motor is used as a drive source for a driving mechanism in a copying machine, a computer, or the like.
The characteristics of a hard magnetic alloy are represented by the second quadrants of hysteresis curves, i.e., demagnetization curves. After magnetization, a hard magnetic alloy is under the reverse magnetic field, i.e., the diamagnetic field, produced by its remanent magnetization, and thus the operating point (the magnetic flux density (B) and demagnetizing field (H) of a material) is represented by a point p on the demagnetization curve thereof. At this point, the product (BH) represents the maximum energy product ((BH).sub.max).
In order to increase the rotational torque of the stepping motor, it is important to use a hard magnetic alloy having the high maximum energy product ((BH).sub.max).
Since the rotational torque of the stepping motor is proportional to the product of the current passing through the stepping motor and the energy (U) of the magnetostatic field produced outside by a hard magnetic alloy, the rotational torque of the stepping motor is increased by increasing the maximum energy product ((BH).sub.max).
In order to increase the maximum energy product ((BH).sub.max) of a hard magnetic alloy, it is necessary to made the shape of a demagnetization curve angular to increase the area surrounded by the demagnetization curve, the magnetic field axis and the magnetization axis. Namely, it is necessary to increase the remanence ratio (Ir/Is) to increase remanent magnetization (Ir) and coercive force (iHc).
Therefore, as a hard magnetic alloy used for a rotor of a HB type motor, a Al--Ni--Co--Fe system magnet, a Nd--Fe--B system sintered magnet, a Nd--Fe--B type bonded magnet, a Sm--Co system sintered magnet, or the like is used.
However, in a stepping motor using a Al--Ni--Co--Fe system magnet, since the Al--Ni--Co--Fe system magnet has a coercive force (iHc) of as low as 1 kOe or less, there is the problem of causing difficulties in attempting to decease the size of the stepping motor.
Although a Nd--Fe--B system sintered magnet and Sm--Co system sintered magnet have high coercive force (iHc) and are thus used for some of small stepping motors, these magnets have the need to sinter a material powder in the production process, and thus have the problem of increasing the production cost of a magnet, thereby increasing the production cost of a stepping motor.
Furthermore, a Nd--Fe--B system bonded magnet is produced by mixing a rubber or plastic binder with a magnetic powder formed by liquid quenching of an alloy melt mainly comprising the Nd.sub.2 Fe.sub.14 B phase or a Fe.sub.3 B--Nd.sub.2 Fe.sub.14 system exchange spring magnetic powder and then compressing molding or injection molding, and thus has low material strength because of inclusion of the binder. There is thus a problem in that the rotor of a stepping motor serving as a driving unit has low strength.
Also, from the viewpoint of material strength, a ribbon having a thickness of about 50 .mu.m or less and obtained by quenching a melt of a Nd--Fe--B system alloy is preferable from the viewpoint of mechanical strength. However, in the use of such a hard magnetic alloy ribbon as the rotor of a stepping motor, many ribbons must be laminated, thereby causing the problem of increasing the production cost of the stepping motor.
A conventional known speaker schematically comprises a pole piece made of iron, a cylindrical yoke provided on the outside of the pole piece with a space therebetween, upper and lower speaker magnet rings provided in the space between the pole piece and the yoke, and a conical diaphragm. In addition, a voice coil is provided in the magnetic gap formed by the speaker magnets, the voice coil being connected to the conical diaphragm. In such a speaker, when a voice current flows from an amplifier to the voice coil, motion accordingly occurs to move the conical diaphragm connected to the voice coil so that sounds can be emitted.
In a conventional speaker, a ferrite magnet or a Al--Ni--Co--Fe system magnet is used as a speaker magnet material, and a Nd--Fe--B system magnet or a Sm--Co system magnet is used as a magnet material having performance superior to the ferrite magnet and Al--Ni--Co--Fe system magnet. Furthermore, many researches have been made for achieving higher performance by using new alloy magnets such as a Sm--Fe--N system alloy and the like.
However, as described above, the Nd--Fe--B system magnet, the Sm--Co system magnet and the Sm--Fe--N system magnet require 10 atomic % or more of Nd or 8 atomic % or more of Sm, and thus have a fault that the production cost is higher than a ferrite magnet and Al--Ni--Co--Fe system magnet because a large amount of expensive rare earth element is used. In addition, the Sm--Co system magnet is a more expensive magnet than the Nd--Fe--B system magnet, and is thus impractical. On the other hand, the Al--Ni--Co--Fe system magnet costs less than a rare earth magnet, but has the problem of excessively low coercive force. There is thus demand for appearance of a speaker magnet material which costs less and has higher hard magnetic properties than a ferrite magnet.